MB Petroelum third party fatality

1
No. 13 July 98
Well Cellar Collapse

Background An incident recently occurred on a PDO
well pulling hoist. The well cellar collapsed
under the strain applied by the hoist mast base
beam resulting in the hoist mast to drop 1 meter.
The well and mast were immediately secured. It
was found that the well cellar had collapsed on
the side where the hoist beam was resting on the
soil. This allowed the stool pad, which sits
between the hoist mast carrier jacking mechanism
and the base beam, to tumble and to drop the mast
1 meter.
Causes of the incident ? The well cellar was of
poor construction quality (from 1977) the
concrete showed protruding gravel from the
cement, pouring faults, no reinforcement and a
thickness of 17 cm. The location also had soft
spots and bumps. ? The foot print of the hoist
base beam was too small for the force to be
adequately distributed and the construction of
the stool pad and jack was also unstable. ?
Location compaction is limited to 20 ton per
square meter with a safety factor of two to
three. The force exerted by the base beam was 60
ton per square meter. Learning Points and Action
to Prevent Recurrence ? The maximum load allowed
per square meter of locations is unknown. As a
rule of thumb, 20 ton per square meter is used
with a built in safety factor of two to three
depending on compacting quality of the
soil. ? The load distribution near a cellar wall
is mainly distributed to the well cellar wall,
(dependent on the quality of compacting of the
soil around the well cellar). ? Old well
cellars and locations are suspected to be of
lower quality due to repairs or oil spill clean
ups. Refills have been carried out without
compacting the soil ( new well cellars have 20
cm wall thickness and are reinforced ). ???The
foot print area of base beam and its stability
were inadequate for the situation, it was
exposed to i.e. heavy hook loads, subsequent
jarring ( shock loads ). The stability and
footprint should be addressed in the work
proposal to suit the location condition. ? Planni
ng of all well related activities should include
a detailed check of the quality and the
strength of the well pad and well cellar.